1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus capable of quantitatively assessing blood flow behaviors using a contrast agent for ultrasound.
2. Description of the Related Art
An ultrasonic diagnostic apparatus is medical image equipment with which tomographic images of soft tissues beneath the body surface are derived from a living body in a noninvasive manner by the ultrasonic pulse echo method, and has been popular in the Departments relating to hearts, abdominal regions, and urinary, and the Department of obstetrics and gynecology. This ultrasonic diagnostic apparatus is, characteristically, smaller in size and lower in price than other types of medical image equipment (e.g., X-ray diagnostic equipment, X-ray CT equipment, MRI diagnostic equipment, nuclear medicine diagnostic equipment), capable of real time display, capable of offering a high level of safety without X-ray exposure, capable of blood flow imaging, and the like. Recently, the contrast echo has become popular with techniques in which more detailed diagnostic images are derived by increasing echo effects of ultrasound thanks to the contrast agent that has been injected into a subject. For example, with cardiac and abdominal organ examinations utilizing the contrast echo, the contrast agent for ultrasound is injected from a low-invasive vein to collect ultrasonic echo signals having intensified by thus injected contrast agent. Generating diagnostic images based on such echo signals allows estimation of the blood flow behaviors in a more detailed manner.
As the imaging technique is established in image diagnosis, the quantitative assessment using the contrast agent has become popular for study. The functional diagnosis through quantitative assessment has become advanced especially in the field of nuclear medicine and others in which pharmaceutical drug study utilizing metabolic functions is active. Exemplarily in a circulatory system, myocardial functional assessments are made utilizing a time intensity curve (TIC: Time Intensity Curve) as a result of plotting time-varying intensity information. Resultantly derived thereby are the time in the contraction phase taken to reach the maximum contraction speed from the last period of expansion, the maximum contraction speed, and the like. Needless to say to the ultrasonic diagnostic apparatus, such a TIC is applicable also to X-ray, X-ray CT, and MRI, all of which are operable using the contrast agent.
As another type of quantitative assessment using the contrast agent, known is a technique of calculating a mean transit time (MTT: Mean Transit Time: in the below, referred to as “MTT”) of the blood flow using the TIC. Such an MTT allows assessment of the blood flow behaviors in organs, and measurement of the flow volume in a quantitative manner. Also in the ultrasonic diagnostic apparatus, quantitative analysis is becoming possible, such as TIC measurement using the contrast agent, and MTT analysis based on the TIC.
FIG. 1 is a diagram for illustrating the MTT. In FIG. 1, in a case of administering the contrast agent on a continual basis, the MTT will be the value to be derived in the following manner. That is, first calculated is an area S enclosed by a saturation value and a TIC between the administration starting time of the contrast agent and the time of reaching the saturation value, and the result is then normalized by the saturation value. Herein, the area calculation and the normalization may be executed in the reverse order, and if this is the case, the TIC may be first normalized by the saturation value to calculate the 5 area.
The issue here is that, the contrast agent used for ultrasonic diagnosis is composed of very-small bubbles, and has such a peculiar physical property that the contrast agent itself may be collapsed and vanished. Thus, simply applying the technique so far used with the MTT in other diagnostic apparatuses cannot realize the quantitative assessment with assured objectivity and accuracy. At present, the MTT in ultrasonic diagnosis is under study quite actively. For example, as to such problems as effects of bioattenuation in any examination using the contrast agent for ultrasound, and varying concentrations of the contrast agent whenever it is a generally known solution therefore is normalization by saturation values.
However, the following problems are not yet solved to put the MTT into practical use in the ultrasonic diagnostic apparatus, for example.
The first problem is the varying MTTs due to uneven beam shapes. To be specific, generally, if the beam shapes are uneven in the depth direction, assessing any two point regions different in beam shape will result in varying volumes available for the very-small bubbles to collapse and vanish. If this is the case, even if a TIC is plotted for regions different in beam shape with respect to organs having the same level of blood flow behaviors without depending on the depth, for example, the resulting saturation values, i.e., maximum values, may still vary. Thus, the normalized TICs show no coincidence, resulting in varying MTTs depending on the depth.
Moreover, using the contrast agent bubbles requires the longer measurement time compared with TIC measurement with other types of medical equipment. In detail, responding to ultrasound exposure, very-small bubbles in thus exposed plane are collapsed and vanished. Therefore, for data acquisition within a sample time required for plotting a TIC, there requires another ultrasound exposure with a wait until the very-small bubbles again fill the same exposed plane. With TIC measurement in diagnosis utilizing flash echo imaging, there requires a data cluster corresponding to each intermittent time interval. Further, if temporal samplings are increased in number to plot a TIC, the cross section under study has to be maintained longer by the time thus increased corresponding to the time interval between the temporal samplings. In an exemplary case where a TIC covering 20 [seconds] is plotted with the temporal samplings of every 1 [second], it simply takes 20 [seconds] with X-ray enhanced and X-ray enhanced CT. On the other hand, in a case of using the contrast agent for ultrasound, it takes                1+2+3+ . . . +18+19+20=210 [seconds]=3.5[minutes]This is because every time scanning is done, the very-small bubbles being the contrast agent collapse and vanish, and thus resetting is required. During that time, the operator thus has to maintain the cross section. If so, however, it is difficult to securely keep the scanning cross section at the same position. There is a possibility of checking the cross section by performing so-called monitor mode scanning with low sound pressure level not to collapse nor vanish the very-small bubbles, however, this requires to retain the probe for a long absolute time. Still more the long-time administration of the contrast agent for ultrasound, and the resulting long-time examination will be disadvantageous for doctors, operators, and patients.        
Furthermore, there may be a case where the very-small bubbles are not fully collapsed or vanished if the contrast agent is of a type hardly collaping or vanishing, if the contrast agent is high in concentration, or if the transmission sound pressure is low. As such, if the bubbles are not fully collapsed or vanished, it results in the following drawbacks.
Firstly, the resulting MTTs will vary depending on the depth. This is because, even if the blood flow rate of the blood flow is constant without depending on the depth in any organ under study, when the selection position of ROI (Region of Interest) changes in the depth direction, the ratio of the very-small bubbles collapsing and vanishing also changes depending on the depth. This is because the transmission ultrasound is attenuated due to reflection and scattering in the process of passing over the very-small bubbles and collapsing those, or in the process of passing through the living body. Secondly, if the very-small bubbles are remained, it means that a signal derived by the next intermittent transmission resultantly includes an offset of the remaining bubbles. Thus, the flow volume of the very-small bubbles, i.e., blood flow rate, cannot be correctly obtained, thereby failing in deriving correct MTTs after all. Thirdly, as described in the foregoing, due to attenuation of the transmission ultrasound or attenuation of the reflected ultrasound, nearly no signal will be returned if the depth reaches at a certain point. Thus, due to a so-called shadowing phenomenon in which shadows are cast on images, there exists regions being not accessible.
The present invention is proposed in consideration of the above circumstances, and an object thereof is to provide an ultrasonic diagnostic apparatus with which effects caused by the depth can be reduced, and MTTs can be derived with assured accuracy and with high reproducibility through short-time scanning.